The present invention relates generally to the field of thermal printing. More particularly, it relates to an improved printhead mechanism featuring a floating printhead that can be used to maintain the alignment between a printhead and a platen.
Thermal printing is a non-impact method of imprinting images on paper and synthetic film. The process applies heat from a thermal printhead onto a thermal material and since thermal printers typically have fewer moving components, they are very reliable, clean, quiet, and easy to maintain. As a result, thermal printing is currently one of the most popular methods used for producing high quality, free format, random, on demand printed images.
The oldest and best known use of thermal printing is the direct thermal printing used in facsimile machines. However, consistently advancing technology has led to a proliferation of important new uses ranging from cash register receipts, tickets, anti-forgery labels, and meteorological plotting. Furthermore, the high image quality that is created with thermal printing makes this technology particularly well suited for printing bar code symbols.
In direct thermal printing, the print mechanism is primarily comprised of a thermal printhead that has a large number of tiny resistors that individually react to convert an electrical impulse into heat. The heat from the thermal printhead creates a reaction with a chemical coating on the thermal material (often paper) to produce an image. The print mechanism for thermal transfer printing is very similar to a direct thermal printer, with the addition of a ribbon supply and take-up mechanism.
The heart of both the direct thermal printing mechanism and thermal transfer printing mechanism is the thermal printhead which provides the heat energy necessary to form an image. Thermal printheads known in the prior art are typically linear thermal array types that are fixed in a secure position and create an image on a media as that media moves past the printhead. In this type of arrangement, the printhead is wide enough to span the entire width of the media that is printed upon, and a support platen behind the media is used to keep the media in contact with the printhead. Since the print element is fixed with regard to the media motion direction, print accuracy is superior. This superior print accuracy is what makes thermal printers one of the most widely used methods of printing machine readable bar codes.
In thermal printing, the most critical factor to the correct operation of the printer is the relationship between the printhead and the support platen behind the media. It is critical that the thermal printing elements are located perpendicular to the centerline of the cylindrical platen and that a proper pressure is maintained between the printhead, ribbon, and media. If the printing elements are not located on the centerline of the cylindrical platen, the elements will not create the same size dot on the printed media.
While thermal printing generally produces a high quality printed product, the fixed printhead disclosed in the prior art cannot accommodate the wide variety of different media thicknesses. Pivoted printheads were developed as a solution to this lack of versatility. In the prior art, a pivoted printhead, like the one disclosed in U.S. Pat. No. 5,051,009, is generally fixed only at a pivot point. Opposite the pivot point, the printhead is spring biased toward the platen roller thus allowing the printhead to move toward or away from the platen as necessary to accommodate varying media thicknesses. As the thickness of the media changes, however, the alignment of the pivoting printhead and the roller changes, negatively affecting print quality. Furthermore, in both a fixed printhead and a pivoting printhead thermal printer, the printhead and the platen roller are forced to be parallel. As a result of the required parallel orientation, if the platen roller and the printhead are even slightly misaligned during manufacture or if the media being used has an inconsistent or uneven thickness, the contact pressure of the printhead across the media will be uneven and the print quality will be inconsistent across the full length of the print line. Accordingly, there is a demand for an improved thermal printhead mechanism.
The present invention overcomes the deficiencies of the prior art by providing an improved printhead mechanism. In accordance with the present invention, the improved printhead mechanism comprises a printhead mounted on a top bracket, a bottom bracket supported by the top bracket, and a fixed spring channel that moveably supports the top bracket. In one embodiment of the invention, the spring channel is fixed to a front mounting plate and supports the top bracket and printhead through the use of a plurality of compression type springs. The compression type springs act to bias the floating printhead toward a platen roller in order to maintain a consistent pressure between the floating printhead and the print media. One embodiment of the present invention also provides for a cam located between the spring channel and a bottom bracket in order to provide a simple mechanism for loading and locking a printer ribbon and print media.
It is another feature of the invention to provide a method of maintaining the alignment between a printhead and a platen roller in a thermal printing device. According to the method, a spring channel is fixed in a permanent location and used to moveably support a top bracket upon which a printhead is mounted. One embodiment of the present method also provides for a plurality of compression type springs to be used to moveably support the top bracket.
By fixing the spring channel in a permanent location and allowing the printhead to float upon the support provided by the spring channel, the present invention is able to accommodate a wide variety of print media thicknesses. In addition, since the present invention allows the printhead to float on the support of the spring channel instead of being fixed or pivoting around a fixed point, it does not need to remain parallel to a platen roller in order maintain the contact pressure of the printhead across an uneven media. Accordingly, the claimed invention is more versatile and provides more consistent print line and overall print quality over a greater variety of medias than the thermal printing mechanisms available in the prior art.
These and other features and advantages of the present invention will be apparent to those skilled in the art upon review of the following detailed description of the drawings and preferred embodiments.